The present invention relates to improved relays and methods for controlling the operation of a power circuit breaker in electrical power transmission and distribution systems. In particular, the present invention relates to reclosing relays for closing and reclosing a power circuit breaker and methods of operation of reclosing relays.
The majority of faults on overhead electric power lines, such as those with heavy tree exposure, are cleared by momentarily de-energizing the line. The momentary power cutoff is accomplished by the circuit breaker, which is opened or tripped by an overload relay when an overload due to a fault is sensed. The condition of the breaker as open or closed is defined by auxiliary "52a" and "52b" contacts on the breaker. The 52a contact is closed only when the breaker is closed, and the 52b contact is closed only when the breaker is open.
Automatically reclosing the breaker, after the fault clears, provides improved system stability and electric service continuity for the consumer. This, in turn, allows higher line loading by decreasing the likelihood of line loss.
The circuit that issues one or more commands (signals) to close the breaker is called a reclosing relay. The term "reclosing relay" as used in the electric power system art refers to a relatively complicated apparatus instead of a mere coil-and-contact device often called a "relay" in other contexts. The breaker may close once in response to the reclosing relay but then be tripped open again by the overload relay or other relay after a short time. In such case it may be desirable for the reclosing relay to reclose the breaker again. A multi-shot reclosing relay can supply a reclose command signal to the circuit breaker more than once in a reclosing sequence. Each issuance of the reclosing command signal is called an "attempt" or "shot". If the breaker is continually being tripped open, the multi-shot reclosing relay will discontinue and prevent further automatic reclosing of the breaker and leave the breaker open. This condition is called "lock-out".
Where most faults are attributable to heavy tree exposure, as in distribution networks, multiple reclosure attempts are common. This is because of low voltage levels, and is desirable considering customer inconvenience during outages.
On distribution and subtransmission networks, it is desirable to delay reclosing to allow motors to drop off and local generators to be separated. On the other hand, faster reclosing in transmission systems minimizes damage and system shock.
Without limiting the scope of applications intended for the invention, one area of its use is now further discussed in the context of the distribution field.
In distribution substations, electric power at a high transmission voltage passes through a stepdown transformer to a distribution bus for distribution to customers at a lower voltage. A circuit breaker is connected on the distribution side of the transformer, and the reclosing is typically thus effected at distribution voltages. Operating power for the substation itself, including protective relays such as overload relays and reclosing relays, is either taken off the transformer through a tertiary winding on the transformer or derived directly from the distribution bus through a separate transformer. There frequently is no back up secure source of power for the substation such as a battery or generator set because of the expense of maintaining them. However, a power outage is not unlikely during a thunderstorm or other fault-producing condition. At such times power can be lost on the transmission side of the substation transformer, just when reclosing operations may be underway in the reclosing relay.
In the prior art, electromechanical reclosing relays have used a synchronous motor with a cam-operated stepping switch arrangement. Electromechanical reclosing relays are expensive and require tedious adjustments in manufacture. Moreover, the operating sequence and timing of electromechanical reclosing relay cannot be readily modified. These problems have led the art to consider some type of electronic reclosing relay arrangement. The electronic reclosing relay approach, however, has hitherto lacked the ability inherent in the electromechanical reclosing relay to recover gracefully from a power outage. (The motor and associated cams and switches of an electromechanical reclosing relay merely stop in place when power is lost, and then resume where they left off in their reclosing operations when power is restored.)
Manifestly, an improved electronic reclosing relay is needed which can be readily adjusted to any desired operating sequence and timing, including both normal and pilot reclose sequences, and which emulates the ability of the electromechanical reclosing relay to retain its position upon power loss during a reclosing operation. Transmission and underground environments present problems analogous to the problems in the distribution environment discussed above. A reclosing relay which is equally applicable to each environment is desirable.
As noted above, electromechanical relays lack flexibility of adjustment. For example, by virtue of their construction, they have an inherent maximum reclosing cycle time which is equal to the revolution time of a camshaft therein. This maximum cycle time may be much longer than the average cycle time that would actually elapse in most fault situations.
The just-noted discrepancy between maximum cycle time and average actual cycle time is a practical problem because of the need of the public and safety personnel to assist injured person (as in an auto accident under a downed power line) as soon as possible. Assistance cannot be safely given unless it is substantially certain that power is removed from the line, i.e. that the maximum cycle time is predictable and has elapsed. Insurance premiums that utilities pay may vary considerably depending on what maximum cycle time their reclosing relays actually have.
Also, where the maximum required cycle time depends on electrical network considerations, the inherent inflexibility of electromechanical reclosing relays forces the utility to carry extra stocks of reclosing relays and requires extra time of utility personnel to identify specific relays for specific locations. An improved reclosing relay is therefore needed which has an adjustable maximum cycle time. In this way improved safety for the public and more economical utility operations can be realized.
Establishing proper reclosing sequences and time interval values for reclosing relays poses a further difficult problem. When the proper sequences and time values are established, unsuccessful reclosures are minimized and reliable availability of electric power for the consuming public is enhanced. Ordinarily, studies by the utility are used as guidelines for the purpose. However, such studies can require expensive equipment and unduly consume expensive personnel time. Test reclosures of breakers during maintenance may constitute data which is irrelevant to a study of reclosing under fault and other conditions. It is therefore difficult to collect reliable data on which to establish time delay values in reclosing relays. There is a need for some convenient improved way of gathering meaningful data on the operations of reclosing relays with minimal equipment addition and burden on personnel.